Common Objective-C Patterns(Chapter 4 of Objective-C Phrasebook)

 1 #import "Singleton.h"
 2 
 3 
 4 @implementation Singleton
 5 static Singleton* instance;
 6 
 7 - (id)init
 8 {
 9     self = [super init];
10     if (self) {
11         // Initialization code here.
12     }
13     
14     return self;
15 }
16 
17 + (id)alloc
18 {
19     @synchronized([Singleton class])
20     {
21         instance = [super alloc];
22         return instance;
23     }
24     return nil;
25 }
26 
27 +(Singleton*)sharedInstance
28 {
29     @synchronized([Singleton class])
30     {
31         if(!instance)
32         {
33             [[self alloc] init];
34         }
35         
36         return instance;
37     }
38     return nil;
39 }
40 
41 - (void)sayHello
42 {
43     NSLog(@"Hello world!");
44 }
45 
46 @end